1. Field of the Invention
The present invention relates to a liquid crystal display device and to a terminal device, and particularly relates to an in-plane switching (IPS) liquid crystal display device wherein high transmittance can be obtained by a simple electrode structure, and to a terminal device that utilizes the liquid crystal display device.
2. Description of the Related Art
Because of their thin profile, light weight, small size, low energy consumption, and other advantages, display devices that use liquid crystals have been widely deployed and used in a range of devices that includes monitors, televisions (TV), and other large terminal devices; notebook-type personal computers, cash dispensers, vending machines, and other mid-sized terminal devices; and personal TVs, PDAs (Personal Digital Assistance), mobile telephones, mobile gaming devices, and other small terminal devices. In the liquid crystal panel that is the primary component of a liquid crystal display device, information is displayed by using an electric field to control the orientation of liquid crystal molecules, and numerous modes have been proposed according to the combination of the type and initial orientation of the liquid crystal molecules, the direction of the electric field, and other characteristics. Among these modes, the modes most often used in a conventional terminal device include an STN (Super Twisted Nematic) mode using a simple matrix structure, and a TN (Twisted Nematic) mode using an active matrix structure. However, a liquid crystal panel that uses these modes has a narrow range of angles in which contrasts can be correctly distinguished, and grayscale inversion occurs outside the optimum viewing position.
This problem of grayscale inversion was relatively insignificant in mobile telephones and other terminal devices when the display content consisted mainly of telephone numbers and other characters. However, with recent technological developments, terminal devices have come to display not only text information, but also large amounts of image information. The visibility of images is therefore severely reduced by grayscale inversion. Liquid crystal panels that use a mode having a wide range of angles at which contrast can be correctly distinguished without the occurrence of grayscale inversion are therefore gradually being installed in terminal devices. Liquid crystal panels having this type of mode are referred to generically as wide-viewing-angle liquid crystal panels, and IPS systems and other horizontal field modes, as well as multi-domain vertical alignment modes, film-compensated TN modes, and the like are applied therein.
In a film-compensated TN mode among the wide-viewing-angle modes that are used in these wide-viewing-angle liquid crystal panels, the viewing angle is improved by bonding a viewing angle compensation film to a TN-mode liquid crystal panel. First of all, in a TN-mode liquid crystal panel, the liquid crystal molecules are aligned parallel to the substrate in advance when a voltage is not being applied. In a TN mode, liquid crystals having uniaxial positive refractive index anisotropy are used, and the direction in which the liquid crystal molecules have a large refractive index is aligned so as to be parallel to the substrate. When a voltage is applied in this state, the liquid crystal molecules stand in the direction perpendicular to the substrate. However, due to the effects of the orienting force of the orientation film that determines the initial orientation, the liquid crystal molecules cannot stand completely upright even when a high voltage is applied near the boundary of the substrate, and the liquid crystal molecules are oriented in a direction that is tilted with respect to the substrate. Specifically, the direction in which the refractive index of the liquid crystal molecules is large is at an angle in relation to the substrate. In this arrangement, when the liquid crystal molecules are observed from the direction in which the refractive index is large, the apparent refractive index of the liquid crystal molecules significantly varies when this direction varies even by a small amount, and the viewing angle is narrowed by this change in the apparent refractive index. Therefore, the viewing angle compensation film in a film-compensated TN mode serves to minimize a change in the apparent refractive index of liquid crystal molecules that are oriented in the aforementioned tilted direction. An example of this film is a compensation film that is positioned so that a discotic compound corresponds with liquid crystal molecules that are oriented in the tilted direction. When this compensation film is used, it is possible to reduce the effects of liquid crystal molecules near the boundary of the substrate when a voltage is applied. Therefore, grayscale inversion can be suppressed and the viewing angle characteristics can be improved.
A multi-domain vertical alignment mode among the aforementioned wide-viewing-angle modes is a system that has domains in which the tilt directions compensate for each other in a vertical-alignment-mode liquid crystal panel that has a vertical alignment state when a voltage is not applied, and in which the liquid crystal molecules are tilted in the direction parallel to the substrate boundary by application of voltage. When the liquid crystal molecules are tilted in only one direction, such as in a vertical alignment mode that is not multi-domain, the viewing angle is narrowed by the effects of liquid crystal molecules that are oriented in a tilted direction, the same as in the aforementioned TN mode when a voltage is applied. Such measures as providing an irregular surface to the substrate are therefore taken in the multi-domain vertical alignment mode so as to create a plurality of domains in which the tilt directions differ from each other. Specifically, liquid crystal molecules that are tilted in a certain direction are optically compensated for by liquid crystal molecules of another domain that are tilted in a different direction, and the viewing angle is improved.
Although the liquid crystal molecules are in a tilted orientation when a voltage is applied, the film-compensated TN mode and the multi-domain vertical alignment mode have common characteristics in which the effects of tilted liquid crystal molecules are optically compensated, and the viewing angle is improved.
In contrast, liquid crystal molecules are not oriented at an angle with respect to the substrate even when a voltage is applied in an IPS or other lateral field mode, and these modes therefore create a fundamentally wide viewing angle.
FIG. 1 is a schematic sectional view of the IPS liquid crystal panel used in a first conventional liquid crystal display device described in Japanese Laid-open Patent Application No. 2002-296611. As shown in FIG. 1, in the first conventional liquid crystal panel 1300, liquid crystal molecules 1202 are provided between a pair of substrates 1200, 1201, and a pair of electrodes 1203, 1204 is disposed on the surface of the substrate 1201 that faces the liquid crystal molecules. The pair of electrodes 1203, 1204 is formed on the same layer and has a parallel electrode-type structure. The dimensions d, w, and L in the conventional IPS system satisfy the relations L/d>1 and L/w>1, wherein d is the cell gap, i.e., the distance between the pair of substrates 1200, 1201; w is the electrode width; and L is the distance between the pair of electrodes. Specifically, the distance between the electrodes is larger than the cell gap and larger than the electrode width. As shown in the drawing, the direction in which the electrodes 1203, 1204 are oriented is defined as the Y direction, and the direction in which the substrates 1200, 1201 are layered is defined as the Z direction. The X direction is defined as the direction that is orthogonal to the Y direction and the Z direction, and the positive direction of each direction is shown in the drawing.
In the first conventional liquid crystal panel thus configured as described in Japanese Laid-open Patent Application No. 2002-296611, a lateral field E is generated between the pair of electrodes 1203, 1204 by applying a different voltage to the pair of electrodes 1203, 1204. The liquid crystal molecules that are positioned between the pair of electrodes 1203, 1204 are driven by this lateral field E. Since the liquid crystal molecules do not rotate within the XY plane and stand in the Z direction, the user does not observe the liquid crystal molecules from the direction in which the liquid crystal molecules have considerable refractive index anisotropy. Specifically, the aforementioned film-compensated TN mode and the multi-domain vertical alignment mode improve the viewing angle characteristics by reducing the effects of liquid crystal molecules that stand in a tilted direction, whereas the IPS system has markedly superior viewing angle characteristics due to the fact that the liquid crystal molecules do not stand in a tilted direction.
FIG. 2 is a schematic sectional view showing the liquid crystal panel used in a second conventional liquid crystal display device described in Japanese Laid-open Patent Application No. 2002-296611. This second conventional liquid crystal panel operates according to a Fringe Field/Switching (FFS) system that is a modified form of the IPS system. As shown in FIG. 2, in the second conventional liquid crystal panel 2300, liquid crystal molecules 2202 are provided between a pair of substrates 2200, 2201, an electrode 2204 as a first electrode is formed on the surface of the substrate 2201 that faces the liquid crystal molecules, an insulation film 2205 is layered on the surface of the electrode 2204 that faces the liquid crystal, and a second electrode 2203 is formed on the insulation film 2205. The electrode 2203 is comb-shaped, the same as the electrode described in relation to the first conventional liquid crystal panel, whereas the electrode 2204 is not patterned in a comb shape. As in the aforementioned first conventional liquid crystal panel, the dimensions d, w, and L in the FFS system satisfy the relations L/d=0 and L/w=0, wherein d is the cell gap, i.e., the distance between substrates 2200, 2201; w is the width of the electrode 2203; and L is the distance between the electrode 2203 and the electrode 2204. In other words, two types of electrodes are formed in different layers, i.e., a comb-shaped electrode 2203 is layered on the electrode 2204 via the insulation film 2205, and the inter-electrode distance L is therefore essentially zero.
In the second conventional liquid crystal panel thus configured as described in Japanese Laid-open Patent Application No. 2002-296611, a lateral field E is generated between the electrodes 2203, 2204 by applying different voltages to the electrodes 2203, 2204. However, the direction of this field differs from that of the aforementioned first liquid crystal panel because of the different electrode structure. Specifically, the electrodes 1203, 1204 in the aforementioned first liquid crystal panel according to the IPS system have a parallel electrode structure in which the electrodes 1203, 1204 are arranged parallel to the Y direction as viewed from the Z direction, and the field is therefore directed in the Y direction. However, since the electrodes 2203, 2204 in the FFS system have a layered electrode structure in which the electrodes 2203, 2204 are layered in the Z direction, there is a strong field component in the Y direction as well as in the Z direction, which is the direction perpendicular to the plane of the substrate, particularly near the edge of the electrode 2203.
As a result, there is almost no driving of liquid crystal molecules positioned above the electrodes in the IPS system even when the liquid crystal molecules 1202 positioned between the electrodes 1203, 1204 are driven, whereas both the liquid crystal molecules positioned between the electrodes 2203 and the liquid crystal molecules 2202 positioned above the electrodes 2203 are driven in the FFS system. Accordingly, when the electrodes are formed from a transparent conductive film composed of indium tin oxide (hereinafter abbreviated as ITO) or the like, the FFS system has advantages in that the electrode portion can also contribute to a display, and the transmittance can be increased relative to an IPS-type liquid crystal panel operating under the same conditions.
It is disclosed that the same effects are obtained by adopting, as another electrode structure in accordance with the FFS system, a parallel electrode structure in which electrodes are formed in the same layer as shown in FIG. 1, rather than a layered electrode structure such as the one shown in FIG. 2. In particular, the effects are obtained by setting the relationships L/d<1 and L/w<1, i.e., keeping the distance between the electrodes smaller than the cell gap and the electrode width.
However, the aforementioned conventional liquid crystal display device has such problems as the following. Specifically, since liquid crystal molecules that are above the electrodes cannot be driven in the conventional IPS system as described above, the transmittance of the liquid crystal panel decreases. Although the conventional electrode-layering-type FFS system differs from the conventional IPS system in that the liquid crystal molecules disposed above the electrodes can also be driven, the electrode structure is complex and requires an increased number of processes to fabricate, thus increasing the cost. These problems, i.e., the reduction in transmittance and the increased cost due to the increased number of fabrication processes, are particularly significant in applications involving small and mid-sized terminal devices.